Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/4532#discussion_r132854814
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/runtime/OutputRowtimeProcessFunction.scala
---
@@ -0,0 +1,62 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.table.runtime
+
+import org.apache.calcite.runtime.SqlFunctions
+import org.apache.flink.api.common.functions.{MapFunction, RichMapFunction}
+import org.apache.flink.configuration.Configuration
+import org.apache.flink.streaming.api.functions.ProcessFunction
+import org.apache.flink.streaming.api.operators.TimestampedCollector
+import org.apache.flink.table.runtime.types.CRow
+import org.apache.flink.util.Collector
+
+/**
+ * Wraps a ProcessFunction and sets a Timestamp field of a CRow as
+ * [[org.apache.flink.streaming.runtime.streamrecord.StreamRecord]]
timestamp.
+ */
+class OutputRowtimeProcessFunction[OUT](
+ function: MapFunction[CRow, OUT],
+ rowtimeIdx: Int)
--- End diff --
in fact the data type is changed to keep the data type unchanged. A time
indicator is externally represented and treated as a `TIMESTAMP` field and only
internally handled as `LONG`. Therefore, we need to convert it into a
`TIMESTAMP` once the result is converted into a `DataStream`.
You are right, that we need to convert all time indicators to `TIMESTAMP`
and not only one. This is currently enforced by the exception that you
observed. Currently users have to cast all but one time indicator attributes to
`TIMESTAMP`. That will also convert them from `long` to `Timestamp`.
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